Clark J.J.

Depositional History and Evolution of the Paso del Indio Site, Vega Baja, Puerto Rico

Jeffrey J. Clark, Jeff Walker, and Reniel Rodrı´guez Ramos
Depositional History and Evolution of the Paso del Indio Site, Vega Baja, Puerto Rico
Geoarchaeology An International Journal (2003)
Volume: 18, Issue: 6, Pages: 625-648

DOI: 10.1002/gea.10083

Potshards discovered during excavation of bridge pilasters for a major expressway over the Rio Indio floodplain, a stream incised within the karsts of north-central Puerto Rico, required large-scale archaeological excavation. Five-meter-deep bridge pilaster excavations in the alluvial valley provide a 4500-year history of deposition. Stratigraphic analysis of the exposed pilaster walls in combination with textural and organic carbon analyses of sediment cores obtained over a much broader area suggest a fluvial system dominated by overbank deposition. Six sequences of alternating light and dark layers of sediment were identified. The darker layers are largely composed of silts and clays, whereas the lighter layers are rich in sandsized sediment. Archaeological evidence indicates the organic-rich dark layers, believed to be buried A horizons, coincide with pre-historic occupation by Cedrosan Saladoid, Elenan Ostionoid, and Chican Ostionoid, extending from A.D. 450 to A.D. 1500. Lighter layers below the dark soil horizons are interpreted as overbank deposits from large magnitude flood events. The floodplain aggraded discontinuously with rapid deposition of sand followed by gradual accumulation of silt, clay, and organic material. An approximately 1-m-thick layer of coarse sand and gravel halfway up the stratigraphic column represents an episode of more frequent and severe floods. Based on radiocarbon ages, this layer aggraded between A.D. 1000 and A.D. 1100, which is well within the Elenan Ostionoid era (A.D. 900–1200). Rates of sedimentation during this period were approximately 8 mm per year, ten times greater than the estimates of sedimentation rates before and after this flood sequence. The cause for the change in deposition is unknown. Nonetheless the Elenan Ostionoid would have had to endure frequent loss of habitation structures and crops during these events.

Effects of land use change on northeastern Puerto Rican rivers

Clark, J. 1997. Effects of land use change on northeastern
Puerto Rican rivers. Phd Dissertation, Johns
Hopkins University, Baltimore, Maryland, 187

The 500 year period considered in this study is sufficiently long that river change may be produced by natural changes in the external controls of river geometry. These controls include changes in the base level and changes in the climate which, like land use, change the supply of sediment to the rivers.

Effects of land-use change on channel morphology in northeastern Puerto Rico

Clark, J. J., and P. R. Wilcock (2000), Effects of land-use change on
channel morphology in northeastern Puerto Rico, Geol. Soc. Am. Bull.,
112(12), 1763– 1777.

Between 1830 and 1950 much of northeastern Puerto Rico was cleared for agriculture. Runoff increased by 50% and sediment supply to the river channels increased by more than an order of magnitude. Much of the land clearance extended to steep valley slopes, resulting in widespread gullying and landslides and a large load of coarse sediments delivered to the stream channels. A shift from agriculture to industrial and residential land uses over the past 50 yr has maintained the elevated runoff while sediment supply has decreased, allowing the rivers to begin removing coarse sediment stored within their channels. The size, abundance, and stratigraphic elevation of in-channel gravel bar deposits increases, channel depth decreases, and the frequency of overbank flooding increases downstream along these channels. This is presumed to be a transient state and continued transport will lead to degradation of the bed in downstream sections as the channel adjusts to the modern supply of water and sediment. A downstream decrease in channel size is contrary to the expected geometry of self-adjusted channels, but is consistent with the presence of partially evacuated sediment remaining from the earlier agricultural period. Reverse (downstream decreasing) channel morphology is not often cited in the literature, although consistent observations are available from areas with similar land-use history. Identification of reverse channel morphology along individual watercourses may be obscured in multiwatershed compilations in which other factors produce a consistent, but scattered downstream trend. Identification of reverse channel morphology along individual streams in areas with similar land-use history would be useful for identifying channel disequilibrium and anticipating future channel adjustments.
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